As an electrolyte material to be used as a polymer electrolyte membrane or a proton conductive polymer to be incorporated in a catalyst layer of an electrode constituting a polymer electrolyte fuel cell, it has been common to employ a polymer obtained by hydrolyzing a copolymer of tetrafluoroethylene (hereinafter referred to as “TFE”) with a perfluorovinyl ether of the formula (A), followed by treatment for conversion to an acid-form to convert —SO2F groups to —SO3H groups. In the formula (A), Y is a fluorine atom or a trifluoromethyl group, m is an integer of from 0 to 3, n is an integer of from 1 to 12, and p is 0 or 1, provided that (m+p)>0.CF2═CF(OCF2CFY)mOp(CF2)nSO2F  (A)
Among such polymers, particularly preferably employed is one obtained by converting a polymer obtainable by copolymerization of TFE with a monomer represented by the formula (B) to (D), to an acid form. In the formulae (B) to (D), q is an integer of from 1 to 8, r is an integer of from 1 to 8, and s is 2 or 3.CF2═CFO(CF2)qSO2F  (B)CF2═CFOCF2CF(CF3)O(CF2)rSO2F  (C)CF2═CF(OCF2CF(CF3))sO(CF2)2SO2F  (D)
However, although the above-mentioned conventional copolymer was excellent in such properties as an ion conductivity to accomplish a high cell output power and durability to make a long term operation possible, it had a problem that the production cost was high, and it could not be produced at low costs. As a large factor for such a high production cost of the conventional copolymer, it may be mentioned that, for example, in the case of a TFE/CF2═CFOCF2CF(CF3)OCF2CF2SO3H copolymer, it is produced by copolymerizing a vinyl ether monomer containing a —SO2F group synthesized by using, as an intermediate, expensive hexafluoropropylene oxide, with TFE.
Whereas, U.S. Pat. No. 4,273,729 discloses a copolymer of a monomer of the formula (2) (monomer (2)) synthesized without using hexafluoropropylene oxide, with TFE.CF2═CFCF2OCF2CF2SO2F  (2)
However, the polymer synthesized for brine electrolysis in an Example (UTILITY EXAMPLE Q) of this patent publication, has an ion exchange capacity of 0.85 (meq/g dry resin) (equivalent weight: 1180), and thus, the ion exchange capacity is inadequate for a fuel cell, whereby it has a problem that the resistance is practically too high. Further, Journal of Applied Polymer Science, Vol. 47, 735-741 (1993) discloses a report on the results of a study of the synthesis of the monomer (2) and copolymerization of TFE with the monomer (2), wherein the relation between the charge composition comprising TFE and the monomer (2) and the obtainable polymer composition, is reported. However, also in this report, the highest ion exchange capacity among the obtained polymers is 0.77 (meq/g dry resin). And, it is stated that rather than the monomer (2), a conventional vinyl ether type monomer having a structure of CF2═CFO— is has a higher reactivity and is more advantageous for the copolymerization.
Namely, the monomer (2) has a lower copolymerization reactivity with TFE than the conventional vinyl ether type monomer, whereby a polymer having a high ion exchange capacity practically useful as an ion exchange membrane, has not heretofore been obtained. In the case of a copolymer using as starting materials a vinyl ether type monomer such as CF2═CFOCF2CF2SO2F or CF2═CFOCF2CF(CF3)OCF2CF2SO2F, and TFE, it is possible to obtain a polymer having an ion exchange capacity of at least 1.1 (meq/g dry resin) easily by means of 2,2′-azobisisobutyronitrile (AIBN), which is hydrocarbon, as the initiator, as disclosed e.g. in Examples of JP-A-60-243292. However, in the case of the monomer (2), polymerization will not substantially proceed by the polymerization by means of AIBN, as disclosed in Comparative Reference Example in this specification.